1. Field of the Invention
This invention relates to thin film capacitors and to a method of making the same.
2. Description of the Prior Art
Thin film capacitors particularly with large capacitive densities and low temperature coefficients of capacitance are highly desired in various areas of microelectronics.
The thin film capacitors employed in microelectronics generally use the dielectrics, such as silicon dioxide, aluminum oxide and tantalum pentoxide. These dielectrics have rather low dielectric constants (about 4 to 20) and therefore the resultant capacitors provide only very moderate capacitance densities. As a result an undesired limitation is placed on the component density of microelectronic circuits employing such capacitors.
Strontium barium niobate crystals of the formula Sr.sub.1-x Ba.sub.x where x equals 0.25-0.75 are known materials. These materials are hereinafter called SBN. The crystal structure of SBN has been described in various prior references which include O. F. Dudnik et al, Soviet Physics--Crystallography, Vol. 15, No. 2, September-October 1970, pages 330-332.
SBN crystals have been successfully used as pyroelectric detectors of infrared radiation (see e.g. A. M. Glass, Journal of Applied Physics vol. 40, November 1969, pages 4699-4713). These detectors employ thin wafers of SBN crystals oriented with the electroded surfaces normal to the polarization vector (c - axis). In this form the SBN element acts as a thermal transducer as well as a capacitor. A change in the crystal temperature due to absorbed radiation power alters its temperature and causes a voltage to develop across the parallel plate capacitor through the pyroelectric effect. For this application the SBN wafers are cut normal to the polarization vector (c axis) in order to maximize the pyroelectric effect. In this orientation, however, the capacitance of the SBN element varies strongly with temperature. Wafers cut parallel to the c - axis show a weak temperature dependence of capacitance, however, such wafers are not useful as infrared detectors because they do not exhibit the pyroelectric effect.
Certain features of dielectric characteristics of SBN films produced in RF sputtering in an oxygen atmosphere are described in the article by V. J. Zhdanov et al, Ferroelectrics, 1980, Vol. 29, pages 219-220. On page 219 of the Zhdanov et al article it is stated that by RF sputtering at an oxygen pressure of 6-9.times.10.sup.-3 Torr there are formed SBN films "the value .epsilon. immediately after manufacturing is insensitive to temperature change over a wide range". However, the Zhdanov et al article indicates also that upon cooling from the temperature employed during sputtering (700.degree. C.-900.degree. C.) the dielectric constant .epsilon. of these SBN film decreases with a decrease in temperature.
Yazaki et al, U.S. Pat. No. 3,823,998 shows a light valve comprising a single crystal plate of SBN as an optically active material and bearing on its major surfaces parallel rows of the conductive strips, the strips on one surface intersecting the strips on the other surface. There is no indication in the Yazaki et al patent that the structure formed by the combination of the crystal and the transparent electrode strips forms a capacitor having a high capacitance density and a low temperature coefficient of capacitance.
GB Patent No. 905253 shows a laminated capacitor comprising a semicrystalline ceramic dielectric having a high dielectric constant. An example of such a dielectric being a semi-crystalline glass containing, besides silicon dioxide, barium niobate and strontium niobate. See, for example, glass composition 88 Table 6, page 16. In FIG. 6 of this patent it is shown that the temperature coefficient of the dielectric constant for this material (88) is quite high.